It is generally desirable to have a switched network which is non-blocking for connecting transmitter stations with receiver stations, that is, one in which any transmitter station can be connected to any free receiver station at any time.
A space switching circuit employing a single stage of crosspoints requires a number of switches equal to the product of the number of transmitter stations, T, and receiver stations, R. This arrangement becomes increasingly impracticable as the number of stations increases because of the need to incorporate a rapidly increasing number of switches. One approach to solving this problem is to use a multistage switching network which can be made non-blocking for a given number of stations by means of specific interconnections. Although this approach reduces the number of switches needed from the T and R product, disadvantages of this approach are that it is not easily expandable and the control system needed to control the switches is increasingly complex. As a second approach, if only a few extra stations are to be added to such a network, a number of additional switches can be added and kept to a minimum only at the expense of redesigning and reconfiguring the interconnections between switches. If this latter approach is to be avoided the existing stages of the multistage circuit must essentially be duplicated, thus requiring some design for add-on stages, which while reducing the redesign necessary, has more switching capacity than may be required.
Another method of providing a non-blocking switched network is to multiplex the signals from the transmitter stations and broadcast the multiplexed signals to the receiver stations and demultiplex the required channel. Various multiplexing schemes have been employed, for example pulse position coding, time domain multiplexing and frequency or wavelength multiplexing. Wavelength multiplexing in optical networks has been recognized as providing, in principle, the means of multiplexing many thousands of optical signals but in practice there are severe obstacles to be overcome in providing an optical network employing a multiplexing of such a large number of frequencies. In an optical fibre network the wavelengths need to be closely spaced to fall within the useable wavelength window of the fibre and the optical sources of the many wavelengths have to be extremely accurately tuned. Similarly the optical detectors of the receivers need to be very accurately tunable to receive only one wavelength with minimum crosstalk from adjacent wavelengths.
It is an object of the present invention to provide a switched optical network which can handle large numbers of signal channels more readily than hitherto possible. It is a further object of the present invention to provide a switched optical network that can be readily expanded to accomodate a relative few extra stations. Accordingly there is provided a switched optical network for connecting signals from any one of a plurality of transmitter stations with any one of a plurality of receiver stations comprising:
at least two optical multiplexing means each responsive to signals from two or more transmitter stations to provide optical multiplexing corresponding to the transmitter station signals;
a plurality of demultiplexing means each responsive to optical multiplexing and a selection signal to output one transmitter station signal to the corresponding receiver station; and
an optical switch means responsive to switch signals to optically couple any one of the optical multiplexers to any one of the demultiplexing means.
The signal from each transmitter station is converted into an optical signal forming part of one of the optical multiplexers coupled to the optical switch means. Because the switch means can optically couple any of the multiplexers to the demultiplexer associated with a chosen receiver station, any transmitter station can be switched to any receiver station. Thus, if, for example, 400 transmitters were to be connected to 400 receivers the respective optical signals could be multiplexed into groups of twenty thereby forming a non-blocking network with reduced complexity to a similar network relying solely on multiplexing all the optical signals onto one by employing a 400 by 400 optical switch.
Any multiplexing method producing an optical multiplex may be used with the network of the present invention. Preferably wavelength multiplexing is used as this technique provides the greatest scope for wideband communication to which optical networks are of particular use. In this case each multiplexing means may include a plurality of optical sources for providing optical power at two or more wavelengths, a plurality of modulators each optically coupled to one of the optical sources and responsive to a signal from a transmitter station to provide an optical signal representative of that signal, and a plurality optical combiners for combining the optical signals to form the optical multiplexing.
Generally, more than one transmitter station signal will modulate an optical carrier of a given wavelength. A separate optical source may be provided for each such modulator or, alternatively, each optical source may provide optical power for more than one modulator. Conveniently every modulator operating at a given wavelength is supplied optical power by a single optical source.
Conveniently all the modulators of the network coupled to an optical source of a given wavelength are housed in a respective distinct unit. Such a unit may have one optical input, for example an optical fibre, for connection to an optical source with an internal optical splitter for splitting the optical power from the optical source to each modulator in the unit. With such an arrangement, an optical network according to the invention can be expanded to accomodate extra transmitters in stages of one unit by providing an additional optical source operating at a wavelength different from the others and coupling the extra unit's modulator outputs to the existing multiplexers.
Another convenient arrangement is to house all the optical modulators of the network whose optical outputs form part of the same multiplexer in a respective, distinct unit. In this case the multiplexing means may conveniently include an optical coupler within the housing having an output for connection to the switching means, the modulators each being connected to an optical source of a different wavelength. Such an optical network can also be conveniently expanded in stages to accommodate extra transmitters by optically coupling the additional modulators to the respective optical sources and optically connecting the additional multiplexer output to the switching means.
If desired, units of different size can be provided in either of the above schemes to provide more flexible growth stages.
The control means will have to be modified on expansion of the network to permit the new interconnections. The switching means may be provided with sufficient optical waveguides for anticipated expansion which could then be achieved readily by splicing each new modulator group to an unused waveguide.
If the size of the network is known beforehand the number of multiplexers and the number of optical signals per multiplexers can be designed to optimise costs by having regard to the relative costs of providing larger and smaller multiplexers and switching means.
The switching means may comprise a parallel bus of optical fibres optically coupled to the multiplexers, each carrying one optical multiplexed signal to a single respective optical space switch, each demultiplexer of the network being connected to one output of every optical switch. That is, each optical space switch is arranged to connect the respective multiplexer to any number of the demultiplexers to which it is connected.
Alternatively, the switch means may have a plurality of optical space switches each having a plurality of inputs equal in number to the number of multiplexers. In this case each multiplexer is optically coupled to an input of every optical space switch, and every demultiplexer is coupled to one output of just one of the optical space switches.
The demultiplexer must be able to extract the required transmitter signal from the multiplexed signal for forward transmission to the corresponding receiver station. In the case of time domain multiplexing this can be achieved by optical demultiplexing methods before the optical signals impinge on an optical detector or by electronic demultiplexing after optical detection. In the case of wavelength multiplexing a tunable optical filter can be used to select the required optical signal from the multiplexed signal.
Where a particular application of the present invention has the optical detectors of the demultiplexers sufficiently close to the optical modulators of the multiplexers, a coherent optical detector may be used employing homodyne or heterodyne detection. There may then be provided a selection means optically coupled to the sources of optical power which is responsive to a selection signal from the control means to connect an optical source with a selected output from an optical space switch. For homodyne detection the optical sources are combined unshifted in wavelength. For heterodyne detection the optical sources are combined with a selected output from the switch means after being frequency shifted a predetermined amount.
It is anticipated that an optical communication network according to the present invention providing interconnections between up to 10,000 transmitter and 10,000 receiver stations can be realised with present technology whilst having a structure of design which allows efficient and economic growth to be achieved in relatively small, incremental steps. When a network utilises wavelength multiplexing, the bandwidth for each multiplexer is limited only by the optoelectronic interface and therefore transmission at G bit rates is expected to be possible.
It will be appreciated by those working in the field of the invention that the control means required for the network can be particularly simple and that the present invention is applicable to both highly centralised and partially distributed control schemes and is not dependent on its operation on the actual control scheme or method employed. It is envisaged that the control means will be electronic and computer controlled but optical control means may be a possibility for future systems.
The present invention is applicable to switched networks other than for use for telecommunications applications. It may be used in interconnecting devices on a multiple processor, shared memory computers or other parallel processing schemes, for example.
Where a large number of multiplexes are employed it may be necessary to use in-line broadband laser amplifiers within the switching means to amplify the multiplexes due to splitting losses involved in optically splitting each multiplex to several optical space switches.